Triazolo-pyridazine derivatives as ligands for GABA receptors

ABSTRACT

1,2,4-triazolo[4,3-b]pyridazine derivatives, represented by                    
     wherein Z represents an optionally substituted tetrahydropyridinyl substituent, are selective ligands for GABA A  receptors, in particular having high affinity for the α2 and/or α3 subunit thereof, are useful in the treatment of anxiety and convulsions.

This is an application under 35 U.S.C. 371 of PCT/GB99/00109 filed Jan. 13, 1999.

The present invention relates to a class of substituted triazolo-pyridazine derivatives and to their use in therapy. More particularly, this invention is concerned with substituted 1,2,4-triazolo[4,3-b]pyridazine derivatives which are ligands for GABA_(A) receptors and are therefore useful in the therapy of deleterious mental states.

Receptors for the major inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), are divided into two main classes: (1) GABA_(A) receptors, which are members of the ligand-gated ion channel superfamily; and (2) GABA_(B) receptors, which may be members of the G-protein linked receptor superfamily. Since the first cDNAs encoding individual GABA_(A) receptor subunits were cloned the number of known members of the mammalian family has grown to include at least six α subunits, four β subunits, three γ subunits, one δ subunit, one ε subunit and two ρ subunits.

Although knowledge of the diversity of the GABA_(A) receptor gene family represents a huge step forward in our understanding of this ligand-gated ion channel, insight into the extent of subtype diversity is still at an early stage. It has been indicated that an α subunit, a β subunit and a γ subunit constitute the minimum requirement for forming a fully functional GABA_(A) receptor expressed by transiently transfecting cDNAs into cells. As indicated above, δ, ε and ρ subunits also exist, but are present only to a minor extent in GABA_(A) receptor populations.

Studies of receptor size and visualisation by electron microscopy conclude that, like other members of the ligand-gated ion channel family, the native GABA_(A) receptor exists in pentameric form. The selection of at least one α, one β and one γ subunit from a repertoire of seventeen allows for the possible existence of more than 10,000 pentameric subunit combinations. Moreover, this calculation overlooks the additional permutations that would be possible if the arrangement of subunits around the ion channel had no constraints (i.e. there could be 120 possible variants for a receptor composed of five different subunits).

Receptor subtype assemblies which do exist include, amongst many others, α1β2γ2, α2β2/3γ2, α3βγ2/3, α2βγ1, α5β3γ2/3, α6βγ2, α6βδ and α4βδ. Subtype assemblies containing an α1 subunit are present in most areas of the brain and are thought to account for over 40% of GABA_(A) receptors in the rat. Subtype assemblies containing α2 and α3 subunits respectively are thought to account for about 25% and 17% of GABA_(A) receptors in the rat. Subtype assemblies containing an α5 subunit are expressed predominantly in the hippocampus and cortex and are thought to represent about 4% of GABA_(A) receptors in the rat.

A characteristic property of all known GABA_(A) receptors is the presence of a number of modulatory sites, one of which is the benzodiazepine (BZ) binding site. The BZ binding site is the most explored of the GABA_(A) receptor modulatory sites, and is the site through which anxiolytic drugs such as diazepam and temazepam exert their effect. Before the cloning of the GABA_(A) receptor gene family, the benzodiazepine binding site was historically subdivided into two subtypes, BZ1 and BZ2, on the basis of radioligand binding studies. The BZ1 subtype has been shown to be pharmacologically equivalent to a GABA_(A) receptor comprising the α1 subunit in combination with a β subunit and γ2. This is the most abundant GABA_(A) receptor subtype, and is believed to represent almost half of all GABA_(A) receptors in the brain.

Two other major populations are the α2βγ2 and α3βγ2/3 subtypes. Together these constitute approximately a further 35% of the total GABA_(A) receptor repertoire. Pharmacologically this combination appears to be equivalent to the BZ2 subtype as defined previously by radioligand binding, although the BZ2 subtype may also include certain α5-containing subtype assemblies. The physiological role of these subtypes has hitherto been unclear because no sufficiently selective agonists or antagonists were known.

It is now believed that agents acting as BZ agonists at α1βγ2, α2βγ2 or α3βγ2 subunits will possess desirable anxiolytic properties. Compounds which are modulators of the benzodiazepine binding site of the GABA_(A) receptor by acting as BZ agonists are referred to hereinafter as “GABA_(A) receptor agonists”. The α1-selective GABA_(A) receptor agonists alpidem and zolpidem are clinically prescribed as hypnotic agents, suggesting that at least some of the sedation associated with known anxiolytic drugs which act at the BZ1 binding site is mediated through GABA_(A) receptors containing the α1 subunit. Accordingly, it is considered that GABA_(A) receptor agonists which interact more favourably with the α2 and/or α3 subunit than with α1 will be effective in the treatment of anxiety with a reduced propensity to cause sedation. Also, agents which are antagonists or inverse agonists at α1 might be employed to reverse sedation or hypnosis caused by α1 agonists.

The compounds of the present invention, being selective ligands for GABA_(A) receptors, are therefore of use in the treatment and/or prevention of a variety of disorders of the central nervous system. Such disorders include anxiety disorders, such as panic disorder with or without agoraphobia, agoraphobia without history of panic disorder, animal and other phobias including social phobias, obsessive-compulsive disorder, stress disorders including post-traumatic and acute stress disorder, and generalized or substance-induced anxiety disorder; neuroses; convulsions; migraine; depressive or bipolar disorders, for example single-episode or recurrent major depressive disorder, dysthymic disorder, bipolar I and bipolar II manic disorders, and cyclothymic disorder; psychotic disorders including schizophrenia; neurodegeneration arising from cerebral ischemia; attention deficit hyperactivity disorder; and disorders of circadian rhythm, eg. in subjects suffering from the effects of jet lag or shift work.

Further disorders for which selective ligands for GABA_(A) receptors may be of benefit include pain and nociception; emesis, including acute, delayed and anticipatory emesis, in particular emesis induced by chemotherapy or radiation, as well as post-operative nausea and vomiting; eating disorders including anorexia nervosa and bulimia nervosa; premenstrual syndrome; muscle spasm or spasticity, e.g. in paraplegic patients; and hearing loss. Selective ligands for GABA_(A) receptors may also be effective as pre-medication prior to anaesthesia or minor procedures such as endoscopy, including gastric endoscopy.

In DE-A-2741763, and in U.S. Pat. Nos. 4,260,755, 4,260,756 and 4,654,343, are described various classes of 1,2,4-triazolo[4,3-b]pyridazine derivatives which are alleged to be useful as anxiolytic agents. The compounds described in DE-A-2741763 and in U.S. Pat. Nos. 4,260,755 and 4,654,343 possess a phenyl substituent at the 6-position of the triazolo-pyridazine ring system. The compounds described in U.S. Pat. No. 4,260,756, meanwhile, possess a heteroaryl moiety at the 6- or 8-position. In none of these publications, however, is there any disclosure or suggestion of 1,2,4-triazolo[4,3-b]pyridazine derivatives wherein the substituent at the 6-position is attached through a directly linked oxygen atom.

EP-A-0085840 and EP-A-0134946 describe related series of 1,2,4-triazolo[3,4-a]phthalazine derivatives which are stated to possess antianxiety activity. However, there is no disclosure nor any suggestion in either of these publications of replacing the benzo moiety of the triazolo-phthalazine ring system with any other functionality.

The present invention provides a class of triazolo-pyridazine derivatives which possess desirable binding properties at various GABA_(A) receptor subtypes. The compounds in accordance with the present invention have good affinity as ligands for the α2 and/or α3 subunit of the human GABA_(A) receptor. The compounds of this invention may interact more favourably with the α2 and/or α3 subunit than with the α1 subunit. Desirably, the compounds of the invention will exhibit functional selectivity in terms of a selective efficacy for the α2 and/or α3 subunit relative to the α1 subunit.

The compounds of the present invention are GABA_(A) receptor subtype ligands having a binding affinity (K_(i)) for the α2 and/or α3 subunit, as measured in the assay described hereinbelow, of 100 nM or less, typically of 50 nM or less, and ideally of 10 nM or less. The compounds in accordance with this invention may possess at least a 2-fold, suitably at least a 5-fold, and advantageously at least a 10-fold, selective affinity for the α2 and/or α3 subunit relative to the α1 subunit. However, compounds which are not selective in terms of their binding affinity for the α2 and/or α3 subunit relative to the α1 subunit are also encompassed within the scope of the present invention; such compounds will desirably exhibit functional selectivity in terms of a selective efficacy for the α2 and/or α3 subunit relative to the α1 subunit.

The present invention provides a compound of formula I, or a salt or prodrug thereof:

wherein

Y represents hydrogen or C₁₋₆ alkyl;

Z represents an optionally substituted tetrahydropyridinyl moiety;

R¹ represents C₃₋₇ cycloalkyl, phenyl, furyl, thienyl or pyridinyl, any of which groups may be optionally substituted; and

R² represents cyano(C₁₋₆)alkyl, hydroxy(C₁₋₆)alkyl, C₃₋₇ cycloalkyl(C₁₋₆)alkyl, propargyl, C₃₋₇ heterocycloalkylcarbonyl(C₁₋₆)alkyl, aryl(C₁₋₆)alkyl or heteroaryl(C₁₋₆)alkyl, any of which groups may be optionally substituted.

The groups Z, R¹ and R² may be unsubstituted, or substituted by one or more, suitably by one or two, substituents. In general, the groups Z, R¹ and R² will be unsubstituted or monosubstituted. Examples of optional substituents on the groups Z, R¹ and R² include C₁₋₆ alkyl, aryl(C₁₋₆)alkyl, pyridyl(C₁₋₆)alkyl, halogen, halo(C₁₋₆)alkyl, cyano, cyano(C₁₋₆)alkyl, hydroxy, hydroxymethyl, C₁₋₆ alkoxy, C₃₋₇ cycloakyl(C₁₋₆)alkoxy, C₃₋₇ cycloalkoxy, amino(C₁₋₆)alkyl, di(C₁₋₆)alkylamino(C₁₋₆)alkyl, di(C₁₋₆)alkylaminocarbonyl(C₁₋₆)alkyl, N-(C₁₋₆)alkylpiperidinyl, pyrrolidinyl(C₁₋₆)alkyl, piperazinyl(C₁₋₆)alkyl, morpholinyl(C₁₋₆)alkyl, di(C₁₋₆)alkylmorpholinyl(C₁₋₆)alkyl and imidazolyl(C₁₋₆)alkyl. Representative substituents include C₁₋₆ alkyl, aryl(C₁₋₆)alkyl, halogen, cyano, hydroxy, hydroxymethyl, C₁₋₆ alkoxy and C₃₋₇ cycloalkyl(C₁₋₆)alkoxy, especially C₁₋₆ alkyl or halogen.

As used herein, the expression “C₁₋₆ alkyl” includes methyl and ethyl groups, and straight-chained or branched propyl, butyl, pentyl and hexyl groups. Particular alkyl groups are methyl, ethyl, n-propyl, isopropyl, tert-butyl and 1,1-dimethylpropyl. Derived expressions such as “C₁₋₆ alkoxy” are to be construed accordingly.

Typical C₃₋₇ cycloalkyl groups include cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl.

The expression “C₃₋₇ cycloalkyl(C₁₋₆)alkyl” as used herein includes cyclopropylmethyl, cyclobutylmethyl, cyclopentylmethyl and cyclohexylmethyl.

Typical aryl groups include phenyl and naphthyl, preferably phenyl.

The expression “aryl(C₁₋₆)alkyl” as used herein includes benzyl, phenylethyl, phenylpropyl and naphthylmethyl.

Suitable heterocycloalkyl groups include azetidinyl, pyrrolidinyl, piperidinyl, piperazinyl, morpholinyl and thiomorpholinyl groups.

Suitable heteroaryl groups include pyridinyl, quinolinyl, isoquinolinyl, pyridazinyl, pyrimidinyl, pyrazinyl, quinoxalinyl, furyl, benzofuryl, dibenzofuryl, thienyl, benzthienyl, pyrrolyl, indolyl, pyrazolyl, indazolyl, oxazolyl, isoxazolyl, thiazolyl, isothiazolyl, imidazolyl, benzimidazolyl, oxadiazolyl, thiadiazolyl, triazolyl and tetrazolyl groups.

The expression “heteroaryl(C₁₋₆)alkyl” as used herein includes furylmethyl, furylethyl, thienylmethyl, thienylethyl, pyrazolylmethyl, oxazolylmethyl, oxazolylethyl, isoxazolylmethyl, thiazolylmethyl, thiazolylethyl, imidazolylmethyl, imidazolylethyl, benzimidazolylmethyl, oxadiazolylmethyl, oxadiazolylethyl, thiadiazolylmethyl, thiadiazolylethyl, triazolylmethyl, triazolylethyl, tetrazolylmethyl, tetrazolylethyl, pyridinylmethyl, pyridinylethyl, pyridazinylmethyl, pyrimidinylmethyl, pyrazinylmethyl, quinolinylmethyl, isoquinolinylmethyl and quinoxalinylmethyl.

The term “halogen” as used herein includes fluorine, chlorine, bromine and iodine, especially fluorine or chlorine.

For use in medicine, the salts of the compounds of formula I will be pharmaceutically acceptable salts. Other salts may, however, be useful in the preparation of the compounds according to the invention or of their pharmaceutically acceptable salts. Suitable pharmaceutically acceptable salts of the compounds of this invention include acid addition salts which may, for example, be formed by mixing a solution of the compound according to the invention with a solution of a pharmaceutically acceptable acid such as hydrochloric acid, sulphuric acid, methanesulphonic acid, fumaric acid, maleic acid, succinic acid, acetic acid, benzoic acid, oxalic acid, citric acid, tartaric acid, carbonic acid or phosphoric acid. Furthermore, where the compounds of the invention carry an acidic moiety, suitable pharmaceutically acceptable salts thereof may include alkali metal salts, e.g. sodium or potassium salts; alkaline earth metal salts, e.g. calcium or magnesium salts; and salts formed with suitable organic ligands, e.g. quaternary ammonium salts.

The present invention includes within its scope prodrugs of the compounds of formula I above. In general, such prodrugs will be functional derivatives of the compounds of formula I which are readily convertible in vivo into the required compound of formula I. Conventional procedures for the selection and preparation of suitable prodrug derivatives are described, for example, in Design of Prodrugs, ed. H. Bundgaard, Elsevier, 1985.

Where the compounds according to the invention have at least one asymmetric centre, they may accordingly exist as enantiomers. Where the compounds according to the invention possess two or more asymmetric centres, they may additionally exist as diastereoisomers. It is to be understood that all such isomers and mixtures thereof in any proportion are encompassed within the scope of the present invention.

Suitably, Y represents hydrogen or methyl, especially hydrogen.

The group Z suitably represents an optionally substituted 1,2,3,6-tetrahydropyridinyl moiety. Suitably, the tetrahydropyridinyl moiety Z is attached through the nitrogen atom in the 1-position thereof. In a typical embodiment, the substituent Z represents 1,2,3,6-tetrahydropyridinyl, either unsubstituted or substituted by C₁₋₆ alkyl, especially methyl. Favourably, Z represents unsubstituted 1,2,3,6-tetrahydropyridin-1-yl.

Examples of typical optional substituents on the group R¹ include methyl, fluoro and methoxy.

Representative values of R¹ include cyclopropyl, phenyl, methylphenyl, fluorophenyl, difluorophenyl, methoxyphenyl, furyl, thienyl, methyl-thienyl and pyridinyl. Suitably, R¹ may represent unsubstituted, monosubstituted or disubstituted phenyl. Particular values of R¹ include phenyl, difluorophenyl and pyridinyl.

Suitable values for the substituent R² in the compounds according to the invention include cyanomethyl, hydroxybutyl, cyclohexylmethyl, propargyl, pyrrolidinylcarbonylmethyl, benzyl, pyrazolylmethyl, isoxazolylmethyl, thiazolylmethyl, thiazolylethyl, imidazolylmethyl, benzimidazolylmethyl, oxadiazolylmethyl, triazolylmethyl, tetrazolylmethyl, pyridinylmethyl, pyridazinylmethyl, pyrimidinylmethyl, pyrazinylmethyl, quinolinylmethyl, isoquinolinylmethyl and quinoxalinylmethyl, any of which groups may be optionally substituted by one or more substituents. Typical values of R² include triazolylmethyl and pyridinylmethyl, either of which groups may be optionally substituted.

Examples of suitable optional substituents on the group R² include C₁₋₆ alkyl, aryl(C₁₋₆)alkyl, pyridyl(C₁₋₆)alkyl, halogen, halo(C₁₋₆)alkyl, cyano, cyano(C₁₋₆)alkyl, hydroxymethyl, C₁₋₆ alkoxy, C₃₋₇ cycloalkyl(C₁₋₆)alkoxy, amino(C₁₋₆)alkyl, di(C₁₋₆)alkylamino(C₁₋₆)alkyl, di(C₁₋₆)alkylaminocarbonyl(C₁₋₆)alkyl, N-(C₁₋₆)alkylpiperidinyl, pyrrolidinyl(C₁₋₆)alkyl, (C₁₋₆)alkyl, morpholinyl(C₁₋₆)alkyl and di(C₁₋₆)alkylmorpholinyl(C₁₋₆)alkyl.

Specific illustrations of particular substituents on the group R² include methyl, ethyl, n-propyl, benzyl, pyridinylmethyl, chloro, chloromethyl, cyano, cyanomethyl, hydroxymethyl, ethoxy, cyclopropylmethoxy, dimethylaminomethyl, aminoethyl, dimethylaminoethyl, dimethylaminocarbonylmethyl, N-methylpiperidinyl, pyrrolidinylethyl, piperazinylethyl, morpholinylmethyl and dimethylmorpholinylmethyl, especially methyl.

Representative values of R² include cyanomethyl, hydroxybutyl, hydroxymethyl-cyclohexylmethyl, propargyl, dimethylaminomethyl-propargyl, dimethylmorpholinylmethyl-propargyl, pyrrolidinylcarbonylmethyl, cyanobenzyl, hydroxymethyl-benzyl, pyrazolylmethyl, dimethyl-pyrazolylmethyl, methyl-isoxazolylmethyl, thiazolylmethyl, methyl-thiazolylmethyl, ethyl-thiazolylmethyl, methyl-thiazolylethyl, imidazolylmethyl, methyl-imidazolylmethyl, ethyl-imidazolylmethyl, benzyl-imidazolylmethyl, benzimidazolylmethyl, methyl-oxadiazolylmethyl, triazolylmethyl, methyl-triazolylmethyl, propyl-triazolylmethyl, benzyl-triazolylmethyl, pyridinylmethyl-triazolylmethyl, cyanomethyl-triazolylmethyl, dimethylaminomethyl-triazolylmethyl, aminoethyl-triazolylmethyl, dimethylaminoethyl-triazolylmethyl, dimethylaminocarbonylmethyl-triazolylmethyl, N-methylpiperidinyl-triazolylmethyl, pyrrolidinylethyl-triazolylmethyl, piperazinylethyl-triazolylmethyl, morpholinylethyl-triazolylmethyl, methyl-tetrazolylmethyl, pyridinylmethyl, methyl-pyridinylmethyl, dimethyl-pyridinylmethyl, ethoxy-pyridinylmethyl, cyclopropylmethoxy-pyridinylmethyl, pyridazinylmethyl, chloro-pyridazinylmethyl, pyrimidinylmethyl, pyrazinylmethyl, quinolinylmethyl, isoquinolinylmethyl and quinoxalinylmethyl.

Specific values of R² include methyl-triazolylmethyl and methyl-pyridinylmethyl.

A favoured value of R² is methyl-triazolylmethyl.

A particular sub-class of compounds according to the invention is represented by the compounds of formula IIA, and salts and prodrugs thereof:

wherein

R¹ is as defined with reference to formula I above;

m is 1 or 2, preferably 1; and

R¹² represents aryl or heteroaryl, either of which groups may be optionally substituted.

Suitable values for R¹² include phenyl, pyrazolyl, isoxazolyl, thiazolyl, imidazolyl, benzimidazolyl, oxadiazolyl, triazolyl, tetrazolyl, pyridinyl, pyridazinyl, pyrimidinyl, pyrazinyl, quinolinyl, isoquinolinyl and quinoxalinyl, any of which groups may be optionally substituted.

Examples of typical substituents on the group R¹² include C₁₋₆ alkyl, aryl(C₁₋₆)alkyl, pyridyl(C₁₋₆)alkyl, halogen, cyano, cyano(C₁₋₆)alkyl, hydroxymethyl, C₁₋₆ alkoxy, C₃₋₇ cycloalkyl(C₁₋₆)alkoxy, di(C₁₋₆)alkylamino(C₁₋₆)alkyl, amino(C₁₋₆)alkyl, di(C₁₋₆)alkylaminocarbonyl(C₁₋₆)alkyl, N-(C₁₋₆)alkylpiperidinyl, pyrrolidinyl(C₁₋₆)alkyl, piperazinyl(C₁₋₆)alkyl and morpholinyl(C₁₋₆)alkyl.

Illustrative values of specific substituents on the group R¹² include methyl, ethyl, n-propyl, benzyl, pyridinylmethyl, chloro, cyano, cyanomethyl, hydroxymethyl, ethoxy, cyclopropylmethoxy, dimethylaminomethyl, aminoethyl, dimethylaminoethyl, dimethylaminocarbonylmethyl, N-methylpiperidinyl, pyrrolidinylethyl, piperazinylethyl and morpholinylmethyl, especially methyl.

Particular values of R¹² include cyanophenyl, hydroxymethyl-phenyl, pyrazolyl, dimethyl-pyrazolyl, methyl-isoxazolyl, thiazolyl, methyl-thiazolyl, ethyl-thiazolyl, imidazolyl, methyl-imidazolyl, ethyl-imidazolyl, benzyl-imidazolyl, benzimidazolyl, methyl-oxadiazolyl, triazolyl, methyl-triazolyl, propyl-triazolyl, benzyl-triazolyl, pyridinylmethyl-triazolyl, cyanomethyl-triazolyl, dimethylaminomethyl-triazolyl, aminoethyl-triazolyl, dimethylaminoethyl-triazolyl, dimethylaminocarbonylmethyl-triazolyl, N-methylpiperidinyl-triazolyl, pyrrolidinylethyl-triazolyl, piperazinylethyl-triazolyl, morpholinylethyl-triazolyl, methyl-tetrazolyl, pyridinyl, methyl-pyridinyl, dimethyl-pyridinyl, ethoxy-pyridinyl, cyclopropylmethoxy-pyridinyl, pyridazinyl, chloro-pyridazinyl, pyrimidinyl, pyrazinyl, quinolinyl, isoquinolinyl and quinoxalinyl.

Specific values of R¹² include methyl-triazolyl and methyl-pyridinyl.

A favoured value of R¹² is methyl-triazolyl.

A particular subset of the compounds of formula IIA above is represented by the compounds of formula IIB, and pharmaceutically acceptable salts thereof:

wherein

R¹ is as defined with reference to formula I above; and

R³ represents hydrogen or methyl.

In relation to formula IIB above, R¹ suitably represents phenyl, difluorophenyl or pyridinyl.

Suitably, R³ represents methyl.

Specific compounds within the scope of the present invention include:

7-(3,6-dihydro-2H-pyridin-1-yl)-6-(2-methyl-2H- 1,2,4-triazol-3-ylmethoxy)-3-phenyl-1,2,4-triazolo[4,3-b]pyridazine;

7-(3,6-dihydro-2H-pyridin-1-yl)-6-(1-methyl-1H-1,2,4-triazol-3-ylmethoxy)-3-phenyl-1,2, 4-triazolo[4,3-b]pyridazine;

3-(2,4-difluorophenyl)-7-(3,6-dihydro-2H-pyridin-1-yl)-6-(2-methyl-2H-1,2,4-triazol-3-ylmethoxy)-3-phenyl- 1,2,4-triazolo[4,3-b]pyridazine;

7-(3,6-dihydro-2H-pyridin-1-yl)-6-(2-methyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(pyridin-3-yl)-1,2,4-triazolo[4,3-b]pyridazine;

7-(3,6-dihydro-2H-pyridin-1-yl)-6-(3-methylpyridin-2-ylmethoxy)-3-(pyridin-3-yl)- 1,2,4-triazolo[4,3-b]pyridazine;

and salts and prodrugs thereof.

Also provided by the present invention is a method for the treatment and/or prevention of anxiety which comprises administering to a patient in need of such treatment an effective amount of a compound of formula I as defined above or a pharmaceutically acceptable salt thereof or a prodrug thereof.

Further provided by the present invention is a method for the treatment and/or prevention of convulsions (eg. in a patient suffering from epilepsy or a related disorder) which comprises administering to a patient in need of such treatment an effective amount of a compound of formula I as defined above or a pharmaceutically acceptable salt thereof or a prodrug thereof.

The binding affinity (K_(i)) of the compounds according to the present invention for the α3 subunit of the human GABA_(A) receptor is conveniently as measured in the assay described hereinbelow. The α3 subunit binding affinity (K_(i)) of the compounds of the invention is ideally 10 nM or less, preferably 2 nM or less, and more preferably 1 nM or less.

The compounds according to the present invention will ideally elicit at least a 40%, preferably at least a 50%, and more preferably at least a 60%, potentiation of the GABA EC₂₀ response in stably transfected recombinant cell lines expressing the α3 subunit of the human GABA_(A) receptor. Moreover, the compounds of the invention will ideally elicit at most a 30%, preferably at most a 20%, and more preferably at most a 10%, potentiation of the GABA EC₂₀ response in stably transfected recombinant cell lines expressing the α1 subunit of the human GABA_(A) receptor.

The potentiation of the GABA EC₂₀ response in stably transfected cell lines expressing the α3 and α1 subunits of the human GABA_(A) receptor can conveniently be measured by procedures analogous to the protocol described in Wafford et al., Mol. Pharmacol., 1996, 50, 670-678. The procedure will suitably be carried out utilising cultures of stably transfected eukaryotic cells, typically of stably transfected mouse Ltk-fibroblast cells.

The compounds according to the present invention exhibit anxiolytic activity, as may be demonstrated by a positive response in the elevated plus maze and conditioned suppression of drinking tests (cf. Dawson et al., Psychopharmacology, 1995, 121, 109-117). Moreover, the compounds of the invention are substantially non-sedating, as may be confirmed by an appropriate result obtained from the response sensitivity (chain-pulling) test (cf Bayley et al., J. Psychopharmacol., 1996, 10, 206-213).

The compounds according to the present invention may also exhibit anticonvulsant activity. This can be demonstrated by the ability to block pentylenetetrazole-induced seizures in rats and mice, following a protocol analogous to that described by Bristow et al. in J. Pharmacol. Exp. Ther., 1996, 279, 492-501.

In order to elicit their behavioural effects, the compounds of the invention will ideally be brain-penetrant; in other words, these compounds will be capable of crossing the so-called “blood-brain barrier”. Preferably, the compounds of the invention will be capable of exerting their beneficial therapeutic action following administration by the oral route.

The invention also provides pharmaceutical compositions comprising one or more compounds of this invention in association with a pharmaceutically acceptable carrier. Preferably these compositions are in unit dosage forms such as tablets, pills, capsules, powders, granules, sterile parenteral solutions or suspensions, metered aerosol or liquid sprays, drops, ampoules, auto-injector devices or suppositories; for oral, parenteral, intranasal, sublingual or rectal administration, or for administration by inhalation or insufflation. For preparing solid compositions such as tablets, the principal active ingredient is mixed with a pharmaceutical carrier, e.g. conventional tableting ingredients such as corn starch, lactose, sucrose, sorbitol, talc, stearic acid, magnesium stearate, dicalcium phosphate or gums, and other pharmaceutical diluents, e.g. water, to form a solid preformulation composition containing a homogeneous mixture of a compound of the present invention, or a pharmaceutically acceptable salt thereof. When referring to these preformulation compositions as homogeneous, it is meant that the active ingredient is dispersed evenly throughout the composition so that the composition may be readily subdivided into equally effective unit dosage forms such as tablets, pills and capsules. This solid preformulation composition is then subdivided into unit dosage forms of the type described above containing from 0.1 to about 500 mg of the active ingredient of the present invention. Typical unit dosage forms contain from 1 to 100 mg, for example 1, 2, 5, 10, 25, 50 or 100 mg, of the active ingredient. The tablets or pills of the novel composition can be coated or otherwise compounded to provide a dosage form affording the advantage of prolonged action. For example, the tablet or pill can comprise an inner dosage and an outer dosage component, the latter being in the form of an envelope over the former. The two components can be separated by an enteric layer which, serves to resist disintegration in the stomach and permits the inner component to pass intact into the duodenum or to be delayed in release. A variety of materials can be used for such enteric layers or coatings, such materials including a number of polymeric acids and mixtures of polymeric acids with such materials as shellac, cetyl alcohol and cellulose acetate.

The liquid forms in which the novel compositions of the present invention may be incorporated for administration orally or by injection include aqueous solutions, suitably flavoured syrups, aqueous or oil suspensions, and flavoured emulsions with edible oils such as cottonseed oil, sesame oil, coconut oil or peanut oil, as well as elixirs and similar pharmaceutical vehicles. Suitable dispersing or suspending agents for aqueous suspensions include synthetic and natural gums such as tragacanth, acacia, alginate, dextran, sodium carboxymethylcellulose, methylcellulose, polyvinyl-pyrrolidone or gelatin.

In the treatment of anxiety, a suitable dosage level is about 0.01 to 250 mg/kg per day, preferably about 0.05 to 100 mg/kg per day, and especially about 0.05 to 5 mg/kg per day. The compounds may be administered on a regimen of 1 to 4 times per day.

The compounds of formula I as defined above may be prepared by a process which comprises reacting a compound of formula III with a compound of formula IV:

wherein Y, Z, R¹ and R² are as defined above; and L¹ represents a suitable leaving group.

The leaving group L¹ is typically a halogen atom, especially chloro.

The reaction between compounds III and IV is conveniently effected by stirring the reactants in a suitable solvent, typically N,N-dimethyl-formamide, in the presence of a strong base such as sodium hydride or lithium bis(trimethylsilyl)amide.

The intermediates of formula III above may be prepared by reacting an aldehyde derivative of formula R¹—CHO with a hydrazine derivative of formula V:

wherein Y, Z and L¹ are as defined above; followed by cyclization of the intermediate Schiff's base thereby obtained.

The reaction between the aldehyde derivative R¹—CHO and compound V is conveniently effected under acidic conditions at an elevated temperature, for example in the presence of a mineral acid such as hydrochloric acid at a temperature in the region of 60° C. Cyclization of the resulting Schiff's base intermediate may then conveniently be carried out by treatment with iron(III) chloride in a suitable solvent, e.g. an alcoholic solvent such as ethanol, at an elevated temperature, typically at the reflux temperature of the solvent; or by treatment with lead(IV) acetate in the presence of acetic acid at an elevated temperature, e.g. a temperature in the region of 60° C.

The intermediates of formula V above may be prepared by reacting the appropriate compound of formula VI:

wherein Y, Z and L¹ are as defined above, and L² represents a suitable leaving group; with hydrazine hydrate, typically in 1,4-dioxane at the reflux temperature of the solvent; followed, if necessary, by separation of the resulting mixture of isomers by conventional means.

The intermediates of formula III above may alternatively be prepared by reacting a compound of formula VI as defined above with a substantially equimolar amount of a hydrazine derivative of formula R¹—CO—NHNH₂ in which R¹ is as defined above; followed, if necessary, by separation of the resulting mixture of isomers by conventional means.

The leaving group L² is typically a halogen atom, especially chloro. In the intermediates of formula VI, the leaving groups L¹ and L² may be the same or different, but are suitably the same, preferably both chloro.

The reaction between the hydrazine derivative R¹—CO—NHNH₂ and compound VI is conveniently effected by heating the reactants in the presence of a proton source such as triethylamine hydrochloride, typically at reflux in an inert solvent such as xylene or 1,4-dioxane.

The reaction between compound VI and hydrazine hydrate or the hydrazine derivative R¹—CO—NHNH₂ will, as indicated above, usually give rise to a mixture of isomeric products depending upon whether the hydrazine nitrogen atom displaces the leaving group L¹ or L². Thus, in addition to the required product of formula III or V, the isomeric compound wherein the Y and Z moieties are reversed will usually be obtained to some extent. For this reason it will generally be necessary to separate the resulting mixture of isomers by conventional methods such as chromatography.

In another procedure, the compounds of formula I as defined above may be prepared by a process which comprises reacting a compound of formula VII with a compound of formula VIII:

wherein Y, Z, R¹ and R² are as defined above; and L³ represents a suitable leaving group.

The leaving group L³ is suitably a halogen atom, typically chloro or bromo.

The reaction between compounds VII and VIII is conveniently effected by stirring the reactants in a suitable solvent, typically N,N-dimethylformamide, in the presence of a strong base such as sodium hydride.

The intermediates of formula VII above may conveniently be prepared by reacting a compound of formula III as defined above with an alkali metal hydroxide, e.g. sodium hydroxide. The reaction is conveniently effected in an inert solvent such as aqueous 1,4-dioxane, ideally at the reflux temperature of the solvent.

In a further procedure, the compounds of formula I as defined above may be prepared by a process which comprises reacting a compound of formula IX with a compound of formula X:

wherein Y, Z, R¹ and R² are as defined above, Alk represents a C₁₋₆ alkyl group, typically n-butyl, and L⁴ represents a suitable leaving group; in the presence of a transition metal catalyst.

The leaving group L⁴ is suitably a halogen atom, e.g. bromo.

A suitable transition metal catalyst of use in the reaction between compounds IX and X comprises dichlorobis(triphenylphosphine)-palladium(II).

The reaction between compounds IX and X is conveniently effected in an inert solvent such as N,N-dimethylformamide, typically at an elevated temperature.

The intermediates of formula IX may be prepared by reacting a compound of formula IV as defined above with a compound of formula XI:

wherein Y, Z, L¹ and L⁴ are as defined above; under conditions analogous to those described above for the reaction between compounds III and IV.

Where they are not commercially available, the starting materials of formula IV, VI, VIII, X and XI may be prepared by methods analogous to those described in the accompanying Examples, or by standard methods well known from the art.

It will be understood that any compound of formula I initially obtained from any of the above processes may, where appropriate, subsequently be elaborated into a further compound of formula I by techniques known from the art. For example, a compound of formula I initially obtained wherein R² is unsubstituted may be converted into a corresponding compound wherein R² is substituted, typically by standard alkylation procedures, for example by treatment with a haloalkyl derivative in the presence of sodium hydride and N,N-dimethylformamide, or with a hydroxyalkyl derivative in the presence of triphenylphosphine and diethyl azodicarboxylate. Furthermore, a compound of formula I initially obtained wherein R² represents cyano(C₁₋₆)alkyl may be converted into the corresponding 3-substituted 1,2,4-triazol-5-yl(C₁₋₆)alkyl analogue by treatment with the appropriate acyl hydrazine derivative in the presence of a base such as sodium methoxide. Similarly, a compound of formula I initially obtained wherein R² represents an optionally substituted propargyl moiety may be converted into the corresponding 1,2,3-triazolylmethyl analogue by treatment with azide anion. A compound of formula I initially obtained wherein the R² substituent is substituted by a halogen atom, e.g. chloro, may be converted into the corresponding compound wherein the R² substituent is substituted by a di(C₁₋₆)alkylamino moiety by treatment with the appropriate di(C₁₋₆)alkylamine, typically with heating in a solvent such as 1,4-dioxane in a sealed tube.

Where the above-described processes for the preparation of the compounds according to the invention give rise to mixtures of stereoisomers, these isomers may be separated by conventional techniques such as preparative chromatography. The novel compounds may be prepared in racemic form, or individual enantiomers may be prepared either by enantiospecific synthesis or by resolution. The novel compounds may, for example, be resolved into their component enantiomers by standard techniques such as preparative HPLC, or the formation of diastereomeric pairs by salt formation with an optically active acid, such as (−)-di-p-toluoyl-d-tartaric acid and/or (+)-di-p-toluoyl-1-tartaric acid, followed by fractional crystallization and regeneration of the free base. The novel compounds may also be resolved by formation of diastereomeric esters or amides, followed by chromatographic separation and removal of the chiral auxiliary.

During any of the above synthetic sequences it may be necessary and/or desirable to protect sensitive or reactive groups on any of the molecules concerned. This may be achieved by means of conventional protecting groups, such as those described in Protective Groups in Organic Chemistry, ed. J. F. W. McOmie, Plenum Press, 1973; and T. W. Greene & P. G. M. Wuts, Protective Groups in Organic Synthesis, John Wiley & Sons, 1991. The protecting groups may be removed at a convenient subsequent stage using methods known from the art.

The following Examples illustrate the preparation of compounds according to the invention.

The compounds in accordance with this invention potently inhibit the binding of [³H]-flumazenil to the benzodiazepine binding site of humnan GABA_(A) receptors containing the α2 or α3 subunit stably expressed in Ltk⁻ cells.

Reagents

Phosphate buffered saline (PBS).

Assay buffer: 10 mM KH₂PO₄, 100 mM KCl, pH 7.4 at room temperature.

[³H]-Flumazenil (18 nM for α1β3γ2 cells; 18 nM for α2β3γ2 cells; 10 nM for α3β3γ2 cells) in assay buffer.

Flunitrazepam 100 μM in assay buffer.

Cells resuspended in assay buffer (1 tray to 10 ml).

Harvesting Cells

Supernatant is removed from cells. PBS (approximately 20 ml) is added. The cells are scraped and placed in a 50 ml centrifuge tube. The) procedure is repeated with a further 10 ml of PBS to ensure that most of the cells are removed. The cells are pelleted by centrifuging for 20 min at 3000 rpm in a benchtop centrifuge, and then fiozen if desired. The pellets are resuspended in 10 ml of buffer per tray (25 cm×25 cm) of cells.

Assay

Can be carried out in deep 96-well plates or in tubes. Each tube contains:

300 μl of assay buffer.

50 μl of [³H]-flumazenil (final concentration for α1β3γ2: 1.8 nM; for α2β3γ2: 1.8 nM; for α3β3γ2: 1.0 nM).

50 μl of buffer or solvent carrier (e.g. 10% DMSO) if compounds are dissolved in 10% DMSO (total); test compound or flunitrazepam (to determine non-specific binding), 10 μM final concentration.

100 μl of cells.

Assays are incubated for 1 hour at 40° C., then filtered using either a Tomtec or Brandel cell harvester onto GF/B filters followed by 3×3 ml washes with ice cold assay buffer. Filters are dried and counted by liquid scintillation counting. Expected values for total binding are 3000-4000 dpm for total counts and less than 200 dpm for non-specific binding if using liquid scintillation counting, or 1500-2000 dpm for total counts and less than 200 dpm for non-specific binding if counting with meltilex solid scintillant. Binding parameters are determined by non-linear least squares regression analysis, from which the inhibition constant K_(i) can be calculated for each test compound.

The compounds of the accompanying Examples were tested in the above assay, and all were found to possess a K_(i) value for displacement of [³H]-flumazenil from the α2 and/or α3 subunit of the human GABA_(A) receptor of 100 nM or less.

EXAMPLE 1 7-(3,6-Dihydro-2H-pyridin-1-yl)-6-(2-methyl-2H-1,2,4-triazol-3-ylmethoxy)-3-phenyl-1,2,4-triazolo[4,3-b]pyridazine a) 4-Bromo-1,2-dihydropyridazine-3,6-dione

A mixture of bromomaleic anhydride (50 g, 283 mmol) and sodium acetate (76.5 g, 562 mmol) in 40% acetic acid/water (750 ml) was treated with hydrazine monohydrate (16.5 ml, 339 mmol) at room temperature under nitrogen. The brown solution was stirred and heated at 100° C. for 18 hours. Upon cooling the mixture was poured into water (1 l) and extracted with ethyl acetate (6×500 ml). The combined extracts were dried (MgSO₄), filtered and evaporated to afford the title pyridazine (20 g, 37%) as an orange solid. ¹H NMR (250 MHz, d₆-DMSO) 7.68 (br s). MS (ES⁺) 193 [MH]⁺, 191 [MH]⁺. This material was used without further purification.

b) 4-Bromo-3,6-dichloropyridazine

A solution of 4-bromo-1,2-dihydropyridazine-3,6-dione (10 g, 52 mmol) in phosphorus oxychloride (100 ml) was stirred and heated at 100° C. under nitrogen for 16 hours. Upon cooling the excess phosphorus oxychloride was removed in vacuo. The residue was azeotroped with toluene (×2), then taken up in dichloromethane/water, The mixture was carefully basified with sodium hydrogen carbonate (solid). It was necessary to dilute the mixture further two get two clear layers. The two layers were separated and the aqueous was extracted with dichloromethane (×3). The combined extracts were dried (Na₂SO₄), filtered and evaporated. The residue was purified by chromatography on silica gel, eluting with dichloromethane to afford the title pyridaziyze (5.0 g, 42%) as a colourless solid. ¹H NMR (250 MHz, CDCl₃) 7.68 (br s). MS (ES⁺) 230 [MH]⁺, 228 [MH]⁺.

c) 3,6-Dichloro-4-(3,6-dihydro-2H-pyridin-1-yl)pyridazine

1,2,3,6-Tetrahydropyridine, (1.04 ml) was added to a stirred solution/suspension of 4-bromo-3,6-dichloropyridazine (2.0 g, 8.8 mmol) and potassium carbonate (2.4 g) in dry DMF (10 ml) at room temperature under nitrogen. The mixture was stirred at room temperature for 5 hours. The reaction was poured into water (100 ml) and extracted with ethyl acetate (×3). The combined extracts were washed with water (200 ml), brine, dried (MgSO₄), filtered and evaporated. The residue was triturated with ether/petroleum ether to afford a white powder (1.96 g, 97%). ¹H NMR (250 MHz, CDCl₃) 6.82 (1H, s), 5.97 (1H, m), 5.80 (1H, m), 3.77 (2H, m), 3.59 (2H, m), 2.38 (2H, m). MS (ES⁺) 232 [MH]⁺.

d) [6-Chloro-5-(3,6-dihydro-2H-pyridin-1-yl)pridazin-3-yl]hydrazine

A mixture of 3,6-dichloro-4-(3,6-dihydro-2H-pyridin-1-yl)pyridazine (1.7 g, 7.4 mmol) and hydrazine hydrate (2.2 ml) in dioxan was stirred and heated at reflux under nitrogen for 24 hours. Upon cooling the dioxan was removed in vacuo and the residue was partitioned between dichloromethane and water. The aqueous was further extracted with dichloromethane (×3). The combined extracts were dried (MgSO₄), filtered and evaporated. The residue was triturated with ether/MeOH and dried in vacuo to afford the title pyridazine (1.5 g, 88%). ¹H NMR (250 MHz, d₆-DMSO) 7.8 (1H, br s), 6.48 (1H, s), 5.76 (2H, m), 4.21 (2H, br s), 3.51 (2H, m), 3.24 (2H, m), 2.16 (2H, m). MS (ES⁺) 226 [MH]⁺, 228 [MH]⁺.

e) N-Benzylidene-N′-[6-chloro-5-(3,6-dihydro-2H-pyridin-1-yl)pyridazin-3-yl]hydrazine

To a suspension of [6-chloro-5-(3,6-dihydro-2H-pyridin-1-yl)-pyridazin-3-yl]hydrazine (1.0 g) in 0.1 M HCl (40 ml) at room temperature was added benzaldehyde (1.05 eq.). The mixture was heated at 50° C. for 2 hours then allowed to cool, diluted with water (100 ml) and filtered. The resulting solid was dried in vacuo, and isolated as a yellow solid (1.25 g, 90%). ¹H NMR (250 MHz, CDCl₃) 11.65 (1H, br s), 8.31 (1H, s), 7.73 (2H, m), 7.38 (2H, m), 7.04 (1H, s), 5.97 (1H, m), 5.83 (1H, m), 3.80 (2H, m), 3.54 (2H, m), 2.40 (2H, m). MS (ES⁺) 314 [MH]⁺, 316 [MH]⁺.

f) 6-Chloro-7-(3,6-dihydro-2H-pyridin-1-yl)-3-phenyl-1,2,4-triazolo[4,3-b]pyridazine

To a solution of the preceding imine (0.5 g) in refluxing ethanol (30 ml) was added a solution of ferric chloride (5 eq.) in ethanol (5 ml) and heating was continued for 5-6 hours. After cooling, the mixture was concentrated in, vacuo, water (80 ml) was added and the aqueous was extracted with dichloromethane (3×50 ml). The combined organic phases were washed with water (2×100 ml) and brine (80 ml), dried (MgSO₄), filtered and concentrated in vacuo. Chromatography on silica eluting with ethyl acetate afforded the desired triazolopyridazine as a white solid (0.39 g, 79%). ¹H NMR (250 MHz, CDCl₃) 8.42 (2H, m), 7.56 (3H, m), 7.45 (1H, s), 5.96 (1H, m), 5.86 (1H, m), 3.70 (2H, m), 3.38 (2H, m), 2.40 (2H, m). MS (ES⁺) 312 [MH]⁺, 314 [MH]⁺.

g) 7-(3,6-Dihydro-2H-pyridin-1-yl)-6-(2-methyl-2H-1,2,4-triazol-3-ylmethoxy)-3-phenyl-1,2,4-triazolo[4,3-b]pyridazine

Sodium hydride (60% dispersion in oil, 23 mg, 0.58 mmol) was added to a solution of (2-methyl-2H-1,2,4-triazol-3-yl)methanol (EP-A-421210) (57 mg, 0.53 mmol) in dry DMF (2 ml) at room temperature under nitrogen. After 1 hour at room temperature a solution of 6-chloro-7-(3,6-dihydro-2H-pyridin-1-yl)-3-phenyl-1,2,4-triazolo[4,3-b]pyridazine (150 mg) in dry DMF (2 ml) was added via syringe. The mixture was stirred at room temperature for 16 hours. Water (20 ml) was added and the solid filtered and dried in vacuo. Recrystallisation from ethyl acetate/petroleum ether afforded the title pyridaziite as a white solid (131 mg, 70%). ¹H NMR (250 MHz, CDCl₃) 8.29 (2H, m), 7.94 (1H, s), 7.52 (3H, m), 7.23 (1H, s), 5.91 (1H, m), 5.80 (1H, m), 5.63 (2H, s), 3.96 (3H, s), 3.71 (2H, m), 3.40 (2H, m), 2.23 (2H, m). MS (ES⁺) 389 [MH]⁺.

EXAMPLE 2 7-(3,6-Dihydro-2H-pyridin-1-yl)-6-(1-methyl-1H-1,2,4-triazol-3-ylmethoxy)-3-phenyl-1,2,4-triazolo[4,3-b]pyridazine

Prepared as described in Example 1, Step (g), using (1-methyl-1H-1,2,4-triazol-3-yl)methanol (EP-A-421210) in place of (2-methyl-2H-1,2,4-triazol-3-yl)methanol. ¹H NMR (250 MHz, CDCl₃) 8.46 (2H, m), 8.05 (1H, s), 7.51 (3H, m), 7.17 (1H, s), 5.89 (1H, m), 5.81 (1H, m), 5.59 (2H, s), 3.94 (3H, s), 3.75 (2H, m), 3.50 (2H, m), 2.30 (2H, m). MS (ES⁺) 389 [MH]⁺.

EXAMPLE 3 3-(2,4-Difluorohenyl)-7-(3,6-dihydro-2H-pyridin-1-yl)-6-(2-methyl-2H-1,2,4-triazol-3-ylmethoxy)-1,2,4-triazolo[4,3-b]pyridazine a) N-[6-Chloro-5-(3,6-dihydro-2H-pyridin-1-yl)pyridazin-3-yl]-N′-(2,4-difluorobenzylidene)hydrazine

Prepared as described in Example 1, Step (e), using 2,4-difluorobenzaldehyde instead of benzaldehyde, and isolated as a yellow solid (1.21 g, 78%). ¹H NMR (250 MHz, CDCl₃) 10.8 (1H, br s), 8.31 (1H, s), 8.13 (2H, m), 7.91 (1H, m), 6.88 (1H, s), 5.98 (1H, m), 5.80 (1H, m), 3.80 (2H, m), 3.54 (2H, m), 2.39 (2H, m). MS (ES⁺) 350 [MH]⁺, 352 [MH]⁺.

b) 6-Chloro-3-(2,4-difluorophenyl)-7-(3 6-dihydro-2H-pyridin-1-yl)-1,2,4-triazolo[4,3-b]pyridazine

Prepared as described in Example 1, Step (f), using the preceding imine in place of the phenyl analogue, and isolated as a white solid (0.22 g, 45%). ¹H NMR (250 MHz, CDCl₃) 7.88 (1H, m), 7.44 (1H, m), 7.09 (2H, s), 5.98 (1H, m), 5.83 (1H, m), 3.70 (2H, m), 3.38 (2H, m), 2.40 (2H, m). MS (ES⁺) 348 [MH]⁺, 350 [MH]⁺.

c) 3-(2,4-Difluorophenyl)-7-(3,6-dihydro-2H-pyridin-1-yl)-6-(2-methyl-2H-1,2,4-triazol-3-ylmethoxy)-1,2,4-triazolo[4,3-b]pyridazine

Prepared as described in Example 1, Step (g), using the preceding triazolo-pyridazine in place of the 3-phenyl analogue. ¹H NMR (250 MHz, CDCl₃) 7.91 (1H, s), 7.83 (1H, m), 7.20 (1H, s), 7.04 (2H, s), 5.88 (1H, m), 5.80 (1H, m), 5.51 (2H, s), 3.88 (3H, s), 3.71 (2H, m), 3.41 (2H, m), 2.23 (2H, m). MS (ES⁺) 425 [MH]⁺.

EXAMPLE 4 7-(3,6-Dihydro-2H-pyridin-1-yl)-6-(2-methyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(pyridin-3-yl)-1,2,4-triazolo[4, 3-b]pyridazine a) N-[6-Chloro-5-(3,6-dihydro-2H-pyridin-1-yl)pyridazin-3-yl]-N′-(pyridin-3-ylmethylene)hydrazine

Prepared as described in Example 1, Step (e), using 3-pyridinecarboxaldehyde instead of benzaldehyde, and isolated as a yellow solid (0.93 g, 67%). ¹H NMR (250 MHz, CDCl₃) 11.85 (1H, br s), 8.96 (1H, m), 8.32 (1H, s), 8.02 (1H, m), 7.34 (1H, m), 7.03 (1H, s), 5.98 (1H, m), 5.82 (1H, m), 3.82 (2H), 3.56 (2H, m), 2.41 (2H, m). MS (ES⁺) 315 [MH]⁺, 317 [MH]⁺.

b) 6-Chloro-7-(3,6-dihydro-2H-pyridin-1-yl)-3-(pyridin-3-yl)-1,2,4-triazolo[4,3-b]pyridazine

Prepared as described in Example 1, Step (f), using the preceding imine in place of the phenyl analogue, and isolated as a white solid (0.319 g, 79%). ¹H NMR (250 MHz, CDCl₃) 9.69 (1H, m), 8.71 (2H, m), 7.49 (1H, s), 7.47 (1H, s), 5.99 (1H, m), 5.84 (1H, m), 3.72 (2H, m), 3.40 (2H, m), 2.40 (2H, m). MS (ES⁺) 313 [MH]⁺, 315 [MH]⁺.

c) 7-(3,6-Dihydro-2H-pyridin-1-yl)-6-(2-methyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(pyridin-3-yl)-1,2,4-triazolo[4,3-b]pyridazine

Prepared as described in Example 1, Step (g), using the preceding triazolo-pyridazine in place of the 3-phenyl analogue. ¹H NMR (250 MHz, CDCl₃) 9.46 (1H, m), 8.70 (2H, m), 7.98 (1H, s), 7.62 (1H, m), 7.45 (1H, s), 5.83 (2H, m), 5.74 (2H, s), 3.95 (3H, s), 3.72 (2H, m), 2.97 (2H, m), 2.15 (2H, m). MS (ES⁺) 390 [MH]⁺.

EXAMPLE 5 7-(3,6-Dihydro-2H-pyridin-1-yl)-6-(3-methylpyridin-2-ylmethoxy)-3-(pyridin-3-yl)-1,2,4-triazolo[4,3-b]pyridazine

Prepared as described in Example 4, Step (c), using (3-methylpyridin-2-yl)methanol in place of (2-methyl-2H-1,2,4-triazol-3-yl)methanol. ¹H NMR (250 MHz, CDCl₃) 9.60 (1H, m), 8.71 (1H, m), 8.62 (1H, m), 8.43 (1H, m), 7.58 (1H, m), 7.41 (1H, m), 7.23 (1H, m), 7.17 (1H s), 5.88 (1H, m), 5.78 (1H, m), 5.65 (2H, s), 3.74 (2H, s), 3.49 (2H, m), 2.44 (3H, s), 2.18 (2H, m). MS (ES⁺) 399 [MH]⁺. 

What is claimed is:
 1. A compound of formula I, or a salt thereof:

wherein Y represents hydrogen or C₁₋₆alkyl; Z represents a tetrahydropyridinyl moiety optionally substituted with one or two independent C₁₋₆alkyl, aryl(C₁₋₆)alkyl, pyridyl(C₁₋₆)alkyl, halogen, halo(C₁₋₆)alkyl, cyano, cyano(C₁₋₆)alkyl, hydroxy, hydroxymethyl, C₁₋₆alkoxy, C₃₋₇cycloalkyl(C₁₋₆)alkoxy, C₃₋₇cycloalkoxy, amino(C₁₋₆)alkyl, di(C₁₋₆)alkylamino(C₁₋₆)alkyl, di(C₁₋₆)alkylaminocarbonyl(C₁₋₆)alkyl, N-(C₁₋₆)alkylpiperidinyl, pyrrolidinyl(C₁₋₆)alkyl, piperazinyl(C₁₋₆)alkyl, morpholinyl(C₁₋₆)alkyl, di(C₁₋₆)alkylmorpholinyl(C₁₋₆)alkyl or imidazolyl(C₁₋₆)alkyl; R1 represents C₃₋₇cycloalkyl, phenyl, furyl, thienyl or pyridinyl, any of which groups may be optionally substituted with one or two independent C₁₋₆alkyl, aryl(C₁₋₆)alkyl, pyridyl(C₁₋₆)alkyl, halogen, halo(C₁₋₆)alkyl, cyano, cyano(C₁₋₆)alkyl, hydroxy, hydroxymethyl, C₁₋₆alkoxy, C₃₋₇cycloalkyl(C₁₋₆)alkoxy, C₃₋₇cycloalkoxy, amino(C₁₋₆)alkyl, di(C₁₋₆)alkylamino(C₁₋₆)alkyl, di(C₁₋₆)alkylaminocarbonyl(C₁₋₆)alkyl, N-(C₁₋₆)alkylpiperidinyl, pyrrolidinyl(C₁₋₆)alkyl, piperazinyl(C₁₋₆)alkyl, morpholinyl(C₁₋₆)alkyl, di(C₁₋₆)alkylmorpholinyl(C₁₋₆)alkyl or imidazolyl(C₁₋₆)alkyl; and R2 represents cyano(C₁₋₆)alkyl, hydroxy(C₁₋₆)alkyl, C₃₋₇cycloalkyl(C₁₋₆)alkyl, propargyl, C₃₋₇heterocycloalkylcarbonyl(C₁₋₆)alkyl, aryl(C₁₋₆)alkyl or heteroaryl(C₁₋₆)alkyl, any of which groups may be optionally substituted with one or two independent C₁₋₆alkyl, aryl(C₁₋₆)alkyl, pyridyl(C₁₋₆)alkyl, halogen, halo(C₁₋₆)alkyl, cyano, cyano(C₁₋₆)alkyl, hydroxy, hydroxymethyl, C₁₋₆alkoxy, C₃₋₇cycloalkyl(C₁₋₆)alkoxy, C₃₋₇cycloalkoxy, amino(C₁₋₆)alkyl, di(C₁₋₆)alkylamino(C₁₋₆)alkyl, di(C₁₋₆)alkylaminocarbonyl(C₁₋₆)alkyl, N-(C₁₋₆)alkylpiperidinyl, pyrrolidinyl(C₁₋₆)alkyl, piperazinyl(C₁₋₆)alkyl, morpholinyl(C₁₋₆)alkyl, di(C₁₋₆)alkylmorpholinyl(C₁₋₆)alkyl or imidazolyl(C₁₋₆)alkyl, wherein aryl represents phenyl or naphthyl, wherein heteroaryl represents pyridinyl, quinolinyl, isoquinolinyl, pyridazinyl, pyrimidinyl, pyrazinyl, quinoxalinyl, furyl, benzofuryl, dibenzofuryl, thienyl, benzthienyl, pyrrolyl, indolyl, pyrazolyl, indazolyl, oxazolyl, isoxazolyl, thiazoyl, isothiazolyl, imidazolyl, benzimidazolyl, oxadiazolyl, thiadiazolyl, triazolyl, or tetrazolyl, and wherein heterocycloalkyl represents azetidinyl, pyrrolidinyl, piperidinyl, piperazinyl, morpholinyl or thiomorpholinyl.
 2. A compound as claimed in claim 1 represented by formula IIA, and salts thereof:

wherein R¹ is as defined in claim 1; m is 1 or 2; and R¹² represents aryl or heteroaryl, either of which groups may be optionally substituted, wherein aryl represents phenyl or naphthyl, wherein heteroaryl represents pyridinyl, quinolinyl, isoquinolinyl, pyridazinyl, pyrimidinyl, pyrazinyl, quinoxalinyl, furyl, benzofuryl, dibenzofuryl, thienyl, benzthienyl, pyrrolyl, indolyl, pyrazolyl, indazolyl, oxazolyl, isoxazolyl, thiazoyl, isothiazolyl, imidazolyl, benzimidazolyl, oxadiazolyl, thiadiazolyl, triazolyl, or tetrazolyl.
 3. A compound as claimed in claim 1 represented by formula IIB, and pharmaceutically acceptable salts thereof:

wherein R¹ is as defined in claim 1; and R³ represents hydrogen or methyl.
 4. A compound selected from: 7-(3,6-dihydro-2H-pyridin-1-yl)-6-(2-methyl-2H-1,2,4-triazol-3-ylmethoxy)-3-phenyl-1,2,4-triazolo[4,3-b]pyridazine; 7-(3,6-dihydro-2H-pyridin-1-yl)-6-(1-methyl-1H-1,2,4-triazol-3-ylmethoxy)-3-phenyl-1,2,4-triazolo[4,3-b]pyridazine; 3-(2,4-difluorophenyl)-7-(3,6-dihydro-2H-pyridin-1-yl)-6-(2-methyl-2H-1,2,4-triazol-3-ylmethoxy)-3-phenyl-1,2,4-triazolo[4,3-b]pyridazine; 7-(3,6-dihydro-2H-pyridin-1-yl)-6-(2-methyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(pyridin-3-yl)-1,2,4-triazolo[4,3-b]pyridazine; 7-(3,6-dihydro-2H-pyridin-1-yl)-6-(3-methylpyridin-2-ylmethoxy)-3-(pyridin-3-yl)-1,2,4-triazolo[4,3-b]pyridazine; and salts thereof.
 5. A pharmaceutical composition comprising a compound of formula I as defined in claim 1 or a pharmaceutically acceptable salt thereof in association with a pharmaceutically acceptable carrier.
 6. A process for the preparation of a compound as claimed in claim 1, which comprises: (A) reacting a compound of formula III with a compound of formula IV:

wherein Y, Z, R¹, and R² are as defined in claim 1, and L¹ represents a suitable leaving group; or (B) reacting a compound of formula VII with a compound of formula VIII:

wherein Y, Z, R¹, and R² are as defined in claim 1, and L³ represents a suitable leaving group; or (C) reacting a compound of formula IX with a compound of formula X:

wherein Y, Z, R¹, and R² are as defined in claim 1, Alk represents a C1-6alkyl group, and L⁴ represents a suitable leaving group; in the presence of a transition metal catalyst.
 7. A method for the treatment of anxiety which comprises administering to a patient in need of such treatment an effective amount of a compound of formula I as defined in claim 1, or a pharmaceutically acceptable salt thereof.
 8. A method for the treatment and/or prevention of convulsions which comprises administering to a patient in need of such treatment an effective amount of a compound of formula I as defined in claim 1, or a pharmaceutically acceptable salt thereof. 